Thermal energy dissipation is a universal task in industry that has largely relied on great quantities of cooling water to satisfy. Common heat rejection processes include steam condensation in thermoelectric power plants, refrigerant condensation in air-conditioning and refrigeration equipment, and process cooling during chemical manufacturing. In the case of power plants and refrigeration systems, it is desired to dissipate thermal energy at the lowest possible temperature with a minimal loss of water to the operating environment for optimum resource utilization.
Where the local environment has a suitable, readily available, low-temperature source of water, e.g., a river, sea, or lake, cooling water can be extracted directly. However, few of these opportunities for cooling are expected to be available in the future because competition for water sources and recognition of the impact of various uses of water sources on the environment are increasing. In the absence of a suitable, readily available coolant source, the only other common thermal sink available at all locations is ambient air. Both sensible heat transfer and latent heat transfer are currently used to reject heat to the air. In sensible cooling, air is used directly as the coolant for cooling one side of a process heat exchanger. For latent cooling, liquid water is used as an intermediate heat-transfer fluid. Thermal energy is transferred to the ambient air primarily in the form of evaporated water vapor, with minimal temperature rise of the air.
These technologies are used routinely in industry, but each one has distinct drawbacks. In the sensible cooling case, air is an inferior coolant compared to liquids, and the resulting efficiency of air-cooled processes can be poor. The air-side heat-transfer coefficient in air-cooled heat exchangers is invariably much lower than liquid-cooled heat exchangers or in condensation processes and, therefore, requires a large heat exchange surface area for good performance. In addition to larger surface area requirements, air-cooled heat exchangers approach the cooling limitation of the ambient dry-bulb temperature of the air used for cooling, which can vary 30° to 40° F. over the course of a day and can hinder cooling capacity during the hottest hours of the day. Air-cooled system design is typically a compromise between process efficiency and heat exchanger cost. Choosing the lowest initial cost option can have negative energy consumption implications for the life of the system.
In latent heat dissipation, the cooling efficiency is much higher, and the heat rejection temperature is more consistent throughout the course of a day since a wet cooling tower will approach the ambient dew point temperature of the air used for cooling instead of the oscillatory dry-bulb temperature of the air used for cooling. The key drawback or problem associated with this cooling approach is the associated water consumption used in cooling, which in many areas is a limiting resource. Obtaining sufficient water rights for wet cooling system operation delays plant permitting, limits site selection, and creates a highly visible vulnerability for opponents of new development.
Prior art U.S. Pat. No. 3,666,246 discloses a heat dissipation system using an aqueous desiccant solution circulated between the steam condenser (thermal load) and a direct-contact heat and mass exchanger in contact with an ambient air flow. In this system, the liquid solution is forced to approach the prevailing ambient dry-bulb temperature and moisture vapor pressure. To prevent excessive drying and precipitation of the hygroscopic desiccant from solution, a portion of the circulating hygroscopic desiccant flow is recycled back to an air contactor without absorbing heat from the thermal load. This results in a lower average temperature in the air contactor and helps to extend the operating range of the system.
The recirculation of unheated hygroscopic desiccant solution is effective for the ambient conditions of approximately 20° C. and approximately 50% relative humidity as illustrated by the example described in U.S. Pat. No. 3,666,246, but in drier, less humid environments, the amount of unheated recirculation hygroscopic desiccant flow must be increased to prevent crystallization of the hygroscopic desiccant solution. As the ambient air's moisture content decreases, the required recirculation flow grows to become a larger and larger proportion of the total flow such that no significant cooling of the condenser is taking place, thereby reducing the ability of the heat dissipation system to cool, in the extreme, to near zero or no significant cooling. Ultimately, once the hygroscopic desiccant is no longer a stable liquid under the prevalent environmental conditions, no amount of recirculation flow can prevent crystallization of the unheated hygroscopic desiccant solution.
Using the instantaneous ambient conditions as the approach condition for the hygroscopic desiccant solution limits operation of the heat dissipation system in U.S. Pat. No. 3,666,246 to a relative humidity of approximately 30% or greater with the preferred MgCl2 hygroscopic desiccant solution. Otherwise, the hygroscopic desiccant may completely dry out and precipitate from solution. This limitation would exclude operation and use of the heat dissipation system described in U.S. Pat. No. 3,666,246 in regions of the world that experience significantly drier weather patterns, less humid air, and are arguably in need of improvements to dry cooling technology.
Additionally, while the heat dissipation system described in U.S. Pat. No. 3,666,246 discloses that the system may alternatively be operated to absorb atmospheric moisture and subsequently evaporate it, the disclosed heat dissipation system design circumvents most of this mode of operation of the heat dissipation system. Assuming that atmospheric moisture has been absorbed into hygroscopic desiccant solution during the cooler, overnight hours, evaporation of water from the hygroscopic desiccant will begin as soon as the ambient temperature begins to warm in the early morning, using the heat dissipation system described in U.S. Pat. No. 3,666,246, since it has no mechanism to curtail excessive moisture evaporation during the early morning transition period and no way to retain excess moisture for more beneficial use later in the daily cycle, such as afternoon, when ambient temperatures and cooling demand are typically higher. Instead, absorbed water in the hygroscopic desiccant in the heat dissipation system will begin evaporating as soon as the hygroscopic desiccant solution's vapor pressure of the heat dissipation system exceeds that of the ambient air, regardless of whether it is productively dissipating thermal energy from the heat load or wastefully absorbing the energy from the ambient air stream.
Improvements have been proposed to these basic cooling systems. Significant effort has gone into hybrid cooling concepts that augment air-cooled condensers with evaporative cooling during the hottest parts of the day. These systems can use less water compared to complete latent cooling, but any increased system performance is directly related to the amount of water-based augmentation, so these systems do not solve the underlying issue of water consumption. Despite the fact that meeting the cooling needs of industrial processes is a fundamental engineering task, significant improvements are still desired, primarily the elimination of water consumption while simultaneously maintaining high-efficiency cooling at reasonable cost.
In summary, there is a need for improved heat dissipation technology relative to current methods. Sensible cooling with air is costly because of the vast heat exchange surface area required and because its heat-transfer performance is handicapped during the hottest ambient temperatures. Latent or evaporative cooling has preferred cooling performance, but it consumes large quantities of water which is a limited resource in some locations.